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How to count element frequencies in an array

Written by Paul Hudson    @twostraws

If you have an array containing various elements and you want to count how often each item appears, you can do so by combining the map() method with a Dictionary initializer.

First, create an array of items:

let items = ["a", "b", "a", "c"]

Second, convert that to an array of key-value pairs using tuples, where each value is the number 1:

let mappedItems = items.map { ($0, 1) }

Finally, create a Dictionary from that tuple array, asking it to add the 1s together every time it finds a duplicate key:

let counts = Dictionary(mappedItems, uniquingKeysWith: +)

That will create the dictionary ["b": 1, "a": 2, "c": 1] because dictionaries are not stored in order – as you can see, it tells us that “a” appeared twice, while the other two appeared once.

Available from iOS 8.0

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